1. Field of the Invention
The present invention pertains to gas-liquid contacting trays of chemical process towers and, more particularly, to an improved downcomer-tray assembly incorporating a catalyst media in conjunction with a raised, active inlet area disposed beneath the downcomer of the tower.
1. History of the Prior Art
Distillation columns are utilized to separate selected components from a multicomponent stream. Generally, such gas-liquid contact columns utilize either trays, packing or combinations thereof. In recent years the trend has been to replace the so-called "bubble caps" by sieve and valve trays in most tray column designs, and the popularity of packed columns, either random (dumped) or structured packing have been utilized in combination with the trays in order to effect improved separation of the components in the stream.
Successful fractionation in the column is dependent upon intimate contact between liquid and vapor phases. Some vapor and liquid contact devices, such as trays, are characterized by relatively high pressure drop and relatively high liquid hold-up. Another type of vapor and liquid contact apparatus, namely structured high efficiency packing, has also become popular for certain applications. Such packing is energy efficient because it has low pressure drop and low liquid hold-up. However, these very properties at times make columns equipped with structured packing difficult to operate in a stable, consistent manner. Moreover, many applications simply require the use of trays.
Fractionation column trays come in two configurations' cross-flow and counter flow. The trays generally consist of a solid tray or deck having a plurality of apertures and are installed on support rings within the tower. In cross-flow trays, vapor ascends through the apertures and contacts the liquid moving across the tray; through the "active" area thereof; in this area liquid and vapor mix and fractionation occurs. The liquid is directed onto the tray by means of a vertical channel from the tray above. This channel is referred to as the Inlet Downcomer. The liquid moves across the tray and exits through a similar channel referred to as the Exit Downcomer. Such downcomers are located where there is a sufficient volume of liquid to effect a liquid-phase, chemical reaction, in the case of catalytic distillation. The location of the downcomers determine the flow pattern of the liquid. If there are two Inlet Downcomers and the liquid is split into two streams over each tray, it is called a two pass tray. If there is only one Inlet and one Outlet Downcomer on opposite sides of the tray, it is called a single pass tray. For two or more passes, the tray is often referred to as a Multipass Tray. The number of passes generally increases as the required (design) liquid rate increases. It is the active area of the tray, however, which is of critical concern.
Not all areas of a tray are active for vapor-liquid contact. For example, the area under the Inlet Downcomer is generally a solid region. To attempt to gain more area of the tray for vapor/liquid contact, the downcomers are often sloped. The maximum vapor/liquid handling capacity of the tray generally increases with an increase in the active or Bubbling Area. There is, however, a limit as to how far one can slope the downcomer(s) in order to increase the Bubbling Area, otherwise the channel will become too small. This can restrict the flow of the liquid and/or restrict the disengagement of vapors retained in the liquid or generated in the downcomers, cause liquid to back up in the downcomer, and thus prematurely limit the normal maximum vapor/liquid handling capacity of the tray. The present invention specifically addresses the problem of restricted disengagement of vapor retained in the liquid, or vapors generated within the downcomer or introduced into the downcomer.
A variation for increasing the Bubbling Area and hence vapor/liquid handling capacity is a Multiple Downcomer (MD) tray. There is usually a plurality of box shaped vertical channels installed in a symmetrical pattern across the tray to direct liquid onto and off of the tray. The downcomers do not extend all the way to the tray below but stop short of the tray by a predetermined distance which is limited by a sufficient space to permit disengagement of any vapor retained in the liquid entering the Exit Downcomer. The downcomer pattern is rotated 90 degrees between successive trays. The bottom of the boxes is solid except for slots that direct the liquid onto the Bubbling Area of the tray below, in between the outlet downcomers of said tray. The MD tray falls into the category of Multipass Trays and is usually used for high liquid rates. The specifics of the present invention where catalyst media is employed in the downcomers to promote a chemical reaction is also applicable to MD trays.
It is well known that the concentration-difference between the vapor and the liquid is the driving force to effect mass transfer. Said concentration-difference can be effected in many ways; some reducing efficiency. For example, as operating pressure increases, descending liquid begins to absorb vapor as it moves across a tray. This is above that normally associated as dissolved gas as governed by Henry's Law and represents much larger amounts of vapor bubbles that are commingled or "entrained" with the liquid. This vapor is not firmly held and is released within the downcomer, and, in fact, the majority of said vapor must be released, otherwise the downcomer can not accommodate the liquid/vapor mixture and will flood thus preventing successful tower operation.
This phenomena is generally deemed to occur when operating pressure is such as to produce a vapor density above about 1.0 lbs/cu. ft. and typically amounts to about 10 to 20% of the vapor by volume.
Similarly, an exothermic reaction in the downcomer will generate vapors from the equilibrium mixture, which must be released. For conventional trays, the released vapor must oppose the descending frothy vapor/liquid mixture flowing over the weir into the downcomer. In many cases, such opposition leads to poor tower operation and premature flooding.
Another serious problem which manifests itself in such operational applications is entrainment of liquid droplets in the ascending vapor. This phenomenon, which is virtually the opposite of the above vapor entrainment, can prevent effective vapor liquid contact. Liquid entrainment is, in one sense, a dynamic flow condition. High velocity vapor flow can suspend descending liquid droplets and prevent their effective passage through the underlying froth mixture zone. It is particularly difficult to prevent this problem when the tower applications require high volume vapor flow in a direction virtually opposite to that of high volume, descending liquid flow.
The technology of gas-liquid contact addresses many performance issues. Examples are seen in several prior art patents, which include U.S Pat. No. 3,959,419, 4,604,247 and 4,597,916, each assigned to the assignee of the present invention and U.S. Pat. No. 4,603,022 issued to Mitsubishi Jukogyo Kabushiki Kaisha of Tokyo, Japan. Another reference is seen in U.S. Pat. No. 4,499,035 assigned to Union Carbide Corporation that teaches a gas-liquid contacting tray with improved inlet bubbling means. A cross-flow tray of the type described above is therein shown with improved means for initiating bubble activity at the tray inlet comprising spaced apart, imperforate wall members extending substantially vertically upwardly and transverse to the liquid flow path. The structural configuration is said to promote activity over a larger tray surface than that afforded by simple perforated tray assemblies. This is accomplished in part by providing a raised region adjacent the downcomer area for facilitating vapor ascension therethrough.
U.S. Patent No. 4,550,000 assigned to Shell Oil Company teaches apparatus for contacting a liquid with a gas in a relationship between vertically stacked trays in a tower. The apertures in a given tray are provided for the passage of gas in a manner less hampered by liquid coming from a discharge means of the next upper tray. This is provided by perforated housings secured to the tray deck beneath the downcomers for breaking up the descending liquid flow. Such advances improve tray efficiency within the confines of prior art structures. Likewise, U.S. Pat. No. 4,543,219 assigned to Nippon Kayaku Kabushiki Kaisha of Tokyo, Japan teaches a baffle tray tower. The operational parameters of high gas-liquid contact efficiency and the need for low pressure loss are set forth. Such references are useful in illustrating the need for high efficiency vapor liquid contact in tray process towers. U.S. Pat. No. 4,504,426 issued to Carl T. Chuang et al. and assigned to Atomic Energy of Canada Limited is yet another example of gas-liquid contacting apparatus. This reference likewise teaches the multitude of advantages in improving efficiency in fractionation and modifications in downcomer-tray designs. The perforated area of the tray is extended beneath the downcomer with between 0 to 25% less perforation area
Yet another reference is seen in U.S. Pat. No. 3,410,540 issued to W. Bruckert in 1968. A downcomer outlet baffle is therein shown to control the discharge of liquid therefrom. The baffle may include either a static seal or dynamic seal. In this regard the openings from the downcomer are sufficiently small to control discharge and may be larger than the tray perforations and of circular or rectangular shape. The transient forces which may disrupt the operation of a downcomer are also more fully elaborated therein. These forces and related vapor-liquid flow problems must be considered for each application in which a downcomer feeds an underlying tray.
A more recent use of distillation columns is for simultaneous or stagewise fractionation in conjunction with a chemical reaction. According to Le Chatelier's Principle well known in the chemical industry, the kinetics of a chemical reaction can be improved by changing the equilibrium of the reactants with the products. For example, the mechanical designs seen in U.S. Pat. Nos. 3,629,478 and 3,634,534 illustrate distillation column reactors with catalyst in the downcomers. In the case of an exothermic reaction with gaseous product being generated, the vapor disengaging space at the entrance to the downcomer could become grossly overloaded causing the entire tower to fail in its intended distillation function. One operational consideration is thus venting of the gases generated within the downcomer, as well as those entrained from the tray deck. Such considerations must be addressed when maximizing tower operational efficiency.
It would be an advantage therefore to provide a method of and apparatus for enhanced downcomer-tray vapor flow utilizing catalyst media therein. Such a downcomer-tray assembly is provided by the present invention wherein a series of uniformly raised, active inlet area panels are secured beneath downcomers containing catalyst bundles therein. The panel has a plurality of apertures, some including flow vanes, disposed beneath the catalyst-downcomer assembly, for providing vapor injection into the liquid flow which is effective in achieving greater vapor-liquid handling capacity. Likewise, the excess vapor from underlying downcomers is vented through the raised, active inlet panel.